1. Field of the Invention
The present invention relates to a Broadband Wireless Access (BWA) communication system adopting a multihop relay scheme. More particularly, the present invention relates to an apparatus and a method for allocating a dedicated access zone to a Relay Station (RS) which performs a distributed scheduling to communicate with a Mobile Station (MS).
2. Description of the Related Art
A fourth generation (4G) communication system, which is a next-generation communication system, aims to provide users with services of various Quality of Service (QoS) levels at a data rate of about 100 Mbps. Particularly, present-day 4G communication systems are advancing in order to guarantee mobility and QoS in Broadband Wireless Access (BWA) communication systems such as Local Area Network (LAN) systems and Metropolitan Area Network (MAN) systems. Representative examples include an Institute of Electrical and Electronics Engineers (IEEE) 802.16d communication system and an IEEE 802.16e communication system.
Both the IEEE 802.16d communication system and the IEEE 802.16e communication system adopt Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) schemes for physical channels. The IEEE 802.16d communication system considers only the fixed status of a current Subscriber Station (SS), that is, takes into account only a single-cell structure without considering the mobility of the SS. In comparison to the IEEE 802.16d communication system, the IEEE 802.16e communication system considers the mobility of the terminal. A mobile terminal is referred to herein as a Mobile Station (MS).
FIG. 1 illustrates a simplified structure of a conventional IEEE 802.16e communication system.
The IEEE 802.16e communication system has a multi-cell structure. For example, the IEEE 802.16e communication system may include a cell 100 and a cell 150. The IEEE 802.16e communication system includes a Base Station (BS) 110 which manages the cell 100, a BS 140 which manages the cell 150, and MSs 111, 113, 130, 151, and 153 which are each located in one or more cells of the system. Between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153, signals are transmitted and received according to the OFDM/OFDMA scheme. Of the MSs 111, 113, 130, 151 and 153, the MS 130 travels within a boundary between the cell 100 and the cell 150. Such a boundary area between the cells is considered a handover area. For example, when the MS 130 moves into the cell 150, managed by the BS 140, while transmitting and receiving signals with the BS 110, its serving BS is changed or handed over from the BS 110 to the BS 140.
Since the signaling is conducted between the fixed BS and the MS using a direct link as shown in FIG. 1, the IEEE 802.16e communication system can easily establish a radio communication link of high reliability between the BS and the MS. However, because of the fixed BS, the IEEE 802.16e communication system has low flexibility regarding its wireless network configuration. Thus, in a radio environment that undergoes several changes of traffic distribution or traffic requirements, it is difficult for the IEEE 802.16e communication system to provide an efficient communication service.
To address these shortcomings, a multihop relay data transmission schemed can be applied to a wireless cellular communication system such as an IEEE 802.16e communication system by using a stationary or mobile Relay Station (RS) or the general MSs. The multihop relay wireless communication system can be used to reconfigure a network in order to promptly handle a communication environment change and therefore operate the entire radio network more efficiently. In addition, the multihop relay wireless communication system can expand the cell service coverage and increase the system capacity. For example, if there is a poor channel condition between the BS and the MS, the multihop relay wireless communication system can establish a multihop relay path via an RS by installing the RS between the BS and the MS, to thus provide the MS with a better radio channel. Also, in a cell boundary in which signal conditions from the BS are weak and subject to greater interference, the multihop relay scheme can provide a high-speed data channel and expand the cell service coverage area.
Now, a structure of a conventional multihop relay wireless communication system that provides expansion of the service coverage area of a BS is illustrated.
FIG. 2 depicts a simplified structure of a multihop relay broadband wireless communication system for the service coverage expansion of the BS.
The multihop relay wireless communication system of FIG. 2 has a multi-cell structure. That is, the multihop relay wireless communication system includes a cell 200 and a cell 240. The multihop relay wireless communication system includes a BS 210 which manages the cell 200 and a BS 250 which manages the cell 240. The multihop relay wireless communication system also includes MSs 211 and 213 in the coverage area of the cell 200, MSs 221 and 223 managed by the BS 210 but out of the coverage area of the cell 200, and an RS 220 which provides multihop relay paths between the BS 210 and the MSs 221 and 223 in a coverage area 230. The multihop relay wireless communication system further includes MSs 251, 253 and 255 in the coverage area of the cell 240, MSs 261 and 263 managed by the BS 250 but out of the coverage area of the cell 240, and an RS 260 which provides multihop relay paths between the BS 250 and the MSs 261 and 263 in a coverage area 270. Between the BSs 210 and 250, the RSs 220 and 260, and the MSs 211, 213, 221, 223, 251, 253, 255, 261 and 263, signals are transmitted and received using the OFDM/OFDMA scheme.
Next, a structure of a multihop relay wireless communication system for increasing system capacity is described.
FIG. 3 illustrates a simplified structure of a conventional multihop relay broadband wireless communication system for increasing system capacity.
The multihop relay wireless communication system of FIG. 3 includes a BS 310 and MSs 311, 313, 321, 323, 331 and 333 located within a cell 300 that are managed by the BS 310. The multihop relay wireless communication system also includes RSs 320 and 330 which provide multihop relay paths between the BS 310 and the MSs 311, 313, 321, 323, 331 and 333. Between the BS 310, the RSs 320 and 330, and the MSs 311, 313, 321, 323, 331 and 333, signals are transmitted and received using the OFDM/OFDMA scheme. The MSs 311, 313, 321, 323, 331 and 333 and the RSs 320 and 333 within the coverage of the cell 300 can transmit and receive signals directly to and from the BS 310.
However, some MSs, such as MSs 321, 323, 331 and 333 near the boundary of the cell 300, are subject to a low Signal to Noise Ratio (SNR) of direct links between the BS 310 and the MSs 321, 323, 331 and 333. The RSs 320 and 330 can increase the effective transfer rate of the MSs and increase the system capacity by providing high-speed data transmission paths to the MSs 321, 323, 331 and 333.
In the multihop relay broadband wireless communication system of FIG. 2 or FIG. 3, the RSs 220, 260, 320 and 330 can be infrastructure RSs installed by a service provider and managed by the BSs 210, 250 and 310 which are aware of the existence of the RSs in advance, or client RSs which serve as SSs (or MSs) or RSs in some cases. The RSs 220, 260, 320, and 330 can be stationary, nomadic (e.g., notebook computer), or mobile like the MS.
The BS in the multihop relay system can schedule the communications with its managing RS and the communications between the RS and the MS. When the BS schedules both the relay link between the BS and the RS as well as the access link between the RS and the MS, the scheduling is referred to as centralized scheduling. In contrast, when the RS directly schedules its MSs without intervention of the BS, the scheduling is referred to as a distributed scheduling.
In the distributed scheduling, the RS may want to communicate with an MS using a dedicated resource. For instance, if a channel between the RS and the MS suffers significant interference from a neighbor node, the RS may want to communicate using a dedicated resource. In this situation, the BS needs to schedule the dedicated resource. More specifically, the RS needs to request a dedicated region from the BS for its own private use, and the BS needs to determine a region corresponding to the RS's request and inform the RS of the determined region. That is, even in the distributed scheduling, the BS needs to schedule an access link (or an access zone) through which the RS and the MS may communicate. Accordingly, there is needed a signaling method for the BS to allocate a Relay (R)-access region to the RS.